In recent years, attention has been drawn to the use of gas assistance with conventional plastic injection molding to attain the product quality and productivity which had been hoped for with structural foam molding. The features of surface quality, lower clamp tonnage, rapid cycle times, weight reduction, material savings and minimization of part distortion or warpage can all be obtained with proper utilization of gas assistance with a conventional plastic injection molding process.
However, as the dimensions of the molded article increase, the gas must do more work to migrate through the volume of the mold cavity to assist in setting up the article within the cavity. If the pressure of the gas is too great as it enters the mold cavity, there is a risk that it may rupture and blow-out the plastic within the mold cavity, i.e. the gas is not contained within the plastic. Thus, there have been practical limitations in the adaptation of gas injection in the plastic molding field.
The U.S. Pat. No. 5,098,637, to Hendry, discloses a method for injection molding hollow plastic articles with pressurized gas which provides for displacement by the gas of a portion of the plastic from the mold cavity into a flow-coupled spill cavity. This feature enables plastic articles of relatively greater dimensions to be successfully molded with the advantages of established gas injection molding techniques.
Some molded parts need a large gas channel to eliminate a second part and secondary operations. Such gas channels may be 3/4 to 1 inches on a thin nominal wall part. Even with a large spill cavity to receive the plastic displaced by a gas charge, such a method is extremely difficult if not impossible.
The reason for this is that with large gas channels on thin nominal wall parts, the large channels act as large elongated runners with very little pressure required to fill the channel and increasing amounts of pressure required in the remote nominal wall area where the plastic flow front slows down and begins to solidify. This results in the nominal wall areas furthest from the gas channels filling last. This causes the effect known as permeation as the gas when injected tries to go to the area of least pressure or the path of least resistance. This results not only in permeation into the nominal walls, but also incomplete evacuation of resin from the area where the gas channel is to be located.